Method of cast-coating paper

ABSTRACT

A METHOD OF CAST-COATING PAPER USING AN AQUEOUS PAPER COATING COMPOSITION WHEREIN THE ADHESIVE CONSISTS ESSENTIALLY OF LATICES, AT LEAST ONE BEING ALKALI-SWELLABLE OR ALKALI-SOLUBLE AND AT LEAST ONE NOT BEING ALKALI-SWELLABLE OR ALKALI-SOLUBLE, THE PH OF THE COMPOSITION BEING WITH THAT THE ALKALI-SWELLABLE OR ALKALI-SOLUBLE LATEX REMAINS IN THE UNSWELLABLE OR UNDISSOLVED CONDITION.

United States Patent Olfice 3,832,216 Patented Aug. 27, 1974 3,832,216METHOD OF CAST-COATING PAPER Philo Burton Cressey, Jr., Gorham, Maine,assignor to Scott Paper Company, Delaware County, Pa. No Drawing. FiledJan. 14, 1972, Ser. No. 218,012 Int. Cl. B44d l /44 US. Cl. 117-64 C 5Claims ABSTRACT OF THE DISCLOSURE A method of cast-coating paper usingan aqueous paper coating composition wherein the adhesive consistsessentially of latices, at least one being alkali-swellable oralkali-soluble and at least one not being alkali-swella'ble oralkali-soluble, the pH of the composition being such that thealkali-swellable or alkali-soluble latex remains in the unswelled orundissolved condition.

BACKGROUND OF THE INVENTION Field of the Invention The present inventionrelates to aqueous paper coating compositions containing latex as thesole adhesive and methods of coating using such compositions, especiallythe cast coating of paper and paperboard.

Description of the Prior Art Paper coating compositions comprise twoessential elements: a pigment and an adhesive or binder which bonds thepigment to the paper base. The cast-coating method of coating papercomprises applying an aqueous composition to a paper web or substrate;pressing the coated surface of the paper against a polished, heated,finishing drum; and maintaining the coated surface of the paper web inintimate non-slipping contact with the drum by adhesion until thecoating has dried, whereupon the paper is separated from the drum. Thisprocess produces the mirror image of the finishing drum surface of thecoated surface of the paper. As a result, the coated paper thus producedhas very high gloss and may actually exhibit increased bulk over theuncoated basestock which results from the introduction of moisture intothe swellable, fibrous bodystock during the coating and finishing steps.

In the cast-coating method as taught by Hart in US. Pat. No. 2,919,205,assigned to the assignee of the present invention, the surface of thefinishing drum is higher than 100 C., e.g. from 120 to 150 C., and thepressure in the nip where the coated paper is pressed against thefinishing drum is greater than the vapor pressure of water at saidtemperature. Additionally, in employing the teachings of Hart, a pool ofboiling water may be maintained in the casting nip. Steam evolving fromthis pool precludes air from the nip and prevents the formation of airbubbles in the coating. Moisture provided by the steam and boiling Wateralso serves to rewet the coating just before contact with the castingdrum, thus promoting good bonding of the coating to the drum surface.See US. Pat. 2,950,989 to Freeman assigned to the assignee of thepresent invention.

As a result of the inventions just described, cast coating speeds farfaster than those taught by the prior art up to that time were madepossible. This speed increase was due to the greater drying capacityavailable when casting drum temperatures were elevated above 100 C. Hartincreased nip pressures above the vaporization point of water at a giventemperature in order to avoid too-sudden vaporization of water in thecoating and basestock at these elevated temperatures and resultantviolent disruption of the coated paper web. Shortly afterward, Freemanfound unexpectedly that steam and boiling water could be utilized in thecasting nip without causing excessive Wetting and subsequent disruptionof the cast paper surface. Although these techniques of Hart and Freemanpermitted far faster operation (250 feet per minute or more) than wasthought possible in the prior art, such extreme conditions of heat andpressure in the presence of nip moisture placed great physical demandsupon the coating employed.

In order to obtain maximum benefits from the Hart and Freeman teachings,coatings were developed which were gelled or coagulated before reachingthe casting nip and thus had greater resistance to the disruptive nipforces. In addition, such coatings were water resistant, and not subjectto wash-01f when brought into contact with a pool of boiling water inthe casting nip. These gelled coatings have been of two basic types:acid-gelled, such as are described in US. Pat. No. 3,078,181; andheat-gelled, of the type taught by US. Pat. No. 3,356,517, both assignedto the assignee of the present invention. Such coatings have producedexcellent end products when employed in conjunction with the teachingsof the Hart and Freeman patents previously cited, and most recentresearch in cast coating has been done with gelled coatings of one typeor the other.

Such gelled coating systems are not without disadvantages however. Asset forth in US. Pat. No. 3,356,517, Column 2, lines 20-41, the acid-gelsystems suffer from sensitivity of pH control and from the corrosiveeffects of the acid employed. The heat-gel systems recommended by thissame US. Pat. No. 3,356,517 and those of US. Pat. No. 3,377,192 arethemselves subject to difficulties, although of a different sort.Extremely careful control must be maintained over coating compositionquantities and method of addition of components in order to makepossible the desired gelation. Temperature must be controlled withinnarrow limits, both during coating formulation and application. Suchcoatings are even sensitive to ambient conditions, especially duringhot, summer weather. The complexity of the coatings and order ofaddition necessitate long make-up times, with nearly constant attentionfrom an operator. Such a time factor adds undesirably to labor costs.

For all of the foregoing reasons it would be highly desirable toeliminate the requirement for gelled coatings in the cast coating art,yet retain the advantages of high operating speeds which such coatingshave made possible when employed in conjunction with high temperatureand pressure conditions. Until the present invention, non-gelledcoatings have been failures under the temperature and pressureconditions of high-speed casting for the reasons previouslycitedinsufiicient cohesiveness to withstand the disruptive forces in thenip, and solubility in the nip liquid. The present invention overcomesthese difficulties and provides a coating composition which may be castat speeds of 300 feet per minute or more at high temperature andpressure conditions but without the need for gelling the coating.

A further problem of the cast-coating art, encompassing both the earlierungelled systems and the more recent gelled ones, has been thedependence of the art upon casein for successful casting. While asdisclosed in the aforementioned patents, 3,356,517 and 3,377,192, it ispossible for the binder in a gellable coating composition to consistsolely of latices, it is still commercially preferred to employ a minorproportion of casein in the adhesive. Casein aids good bonding of thecoating surface to the casting drum and it gives up water readily whenheated. These characteristics prevent the formation of pits on the castsurface as moisture transpires through the back of the web while it isin contact with the casting drum. The use of casein, however, isattended by a number of drawbacks. One is that in order to make up acasein-based composition, it is necessary first to dissolve 3 the caseinin water which is then heated gently to a temperature not to exceed 60C. Careful control of the heating is required in order not to degradethe casein during the heating step. An alkali is then added to thecasein solution, and the resulting solution is then added to adispersion of the pigment.

Another disadvantage of systems containing casein, affecting gelled andungelled systems alike, is that the price and supply of casein aresubject to wide fluctuations. At present, casein for paper coatingpurposes is becoming virtually unobtainable. It is obvious that for easeof process operation and economic reasons it would be a greatimprovement in the cast-coating art if casein could be totallyeliminated in all types of casting systems.

Attempts to replace the casein with a synthetic resin latex or emulsionhave not heretofore met with success in ungelled cast-coatingcompositions because such formulations are not sufficiently water-lovingto cast well on the drum; that is, the coating in its wet condition willnot properly adhere to the surface of the finishing drum. In thelanguage of the art, it will not wet out. This adhesion problem isworsened as operating speeds are increased and additional heat isnecessitated to dry the coating partially prior to contact with thecasting drum. Most latices cross-link or coalesce under the effects ofsuch heat and the coated paper surface is even less hydrophilic when itreaches the casting drum after heating than it is at ambienttemperature.

One type of coating composition which has successfully eliminated theneed for casein in the paper coating art apart from east coating is thatembodying the use of the so-called alkali-swellable or alkali-solublelatices. See U.S. Pat. No. 3,513,121 granted May 19, 1970. This patentdiscloses a composition for coating paper comprising an aqueousdispersion of pigment, an adhesive composition comprising a blend ofcopolymer latices one of which is capable of being rendered soluble byalkali and one of which is not capable of being rendered soluble byalkali, and sufficient alkali to dissolve the alkali soluble latex. Thiscoating composition, however, has not shown itself to be useful in castcoating since the binder component becomes coalesced in the alkalinemedium prior to reaching the casting drum.

An adhesive composition for use in coating paper comprising a blend oflatices, one of which is capable of being swelled by alkali and theother of which is insoluble and not swellable to any substantial extentin aqueous alkali has heretofore been proposed. See Canadian Pat.862,188. Another patent which discloses the use of an alkali-swellablelatex as an adhesive for a paper coating composition is U.S. Pat.3,409,569. These patents teach, however, that in order to obtain thenecessary coating viscosity sufficient alkali must be added to the papercoating composition to swell the latex. When such a coating compositionis employed in the cast coating method of the present invention, thebinder of the coating composition has shown a tendency to absorb water,swell, and thus coalesce prior to the time the coated web comes intocontact with the heated casting drum surface. The coating thereforefails to cast when brought into contact with the finishing surface ofthe casting drum.

SUMMARY OF THE INVENTION It is therefore an object of this invention toprovide an aqueous coating composition in which the adhesive componentconsists of latices of synthetic organic polymers.

It is another object of the invention to provide aqueous cast-coatingcompositions which may be processed at high speeds and which need not begelled, in which the adhesive consists of a blend of alkali-swellable oralkali-soluble and non-alkali-swellable or non-alkali-soluble polymers.

The present inventor has found that if the alkali-swellable oralkali-soluble latex-containing coating compositions of theabove-described patents are not neutralized and are used in their acidor non-swelled or undissolved condition, excellent cast coating resultsare obtained. This is completely opposed to all the teachings of theart, as indicated by the above-cited patents, which have consistentlyrecommended the use of alkali-swellable or alkali-soluble materials intheir alkali state only. Thus, it may be seen that although workers ofextraordinary skill in the paper-coating art have been working withalkali-swellable or alkali-soluble latices for more than five years,none of them has perceived any advantage in the cast coating of paper toemploy these latices in a manner opposite to that of their intended use.

Accordingly, the present invention comprises an aqueous coatingcomposition in which the adhesive component consists essentially of amixture of a latex which is not capable of being swelled or renderedsoluble by alkali and a latex which is capable of being swelled orrendered soluble by alkali. It will, of course, be appreciated that theinvention is not limited to mixtures of only two latices of which one isalkali swellable or soluble and the other is unaffected by alkali. Theuse of three or more different latices can be employed to adjust theproperties of the adhesive as desired. Thus, the adhesive component ofthe coating composition of the present invention broadly comprises amixture of latices of which at least one is insoluble in aqueous alkalisolution and at least one is swellable or soluble in aqueous alkalisolution.

Further in accordance with the present invention, the aqueous coatingcomposition is maintained at a pH which is acidic throughout the coatingand casting process. At acidic pH the alkali-swellable or alkali-solublelatex remains in the unswelled or undissolved condition, and thusuncoalesced until the coated paper surface reaches the casting drum. Allof the latices of the present invention which the present inventor hasstudied become water swollen and coalesced at pHs above 7, and many ofthe most effective systems have shown a tendency to coalesce at a pH ofabout 6.5 and above. It is theoretically possible that a polymer couldbe designed which would remain in its free-acid state until a pHsubstantially above 7 was reached, but none of these has come to theattention of the inventor. By maintaining the pH of the entire coatingcomposition at an acidic level the minor proportion of alkaliswellableor alkali-soluble latex will remain hydrophilic, thus permitting thecoated paper surface to adhere firmly to the casting drum. Once thecoated paper surface is cast firmly against the drum, coatingscontaining the latices of the present invention give up water readily,thus permitting rapid drying, good release from the drum and highoperating speeds.

According to another aspect of the invention, a method of cast coatingpaper is provided wherein the coating composition is applied to thepaper web and directly conveyed to the casting nip without the necessityof gelling or flocculating the coating.

The adhesive components of the paper coating composition of the presentinvention comprises a major proportion of non-alkali-swellable orsoluble latex and a minor proportion of alkali-swellable or solublelatex. Although spoken of as a part of the adhesive component of thepaper coating, the alkali-swellable or soluble latex is requiredprimarily for its hydrophilic properties in its acidic state, and notfor strength-imparting properties. While these alkali-swellable oralkali-soluble materials are strong adhesives in the alkaline conditionin which they were intended to be used, they have relatively littlestrength at acid pH. Most of the binding strength of the adhesivecomponent in the coating composition must therefore be provided by thenon-alkali-swellable or nonalkali-soluble latex component of the binder.Too high a percentage of the alkali-swellable or soluble material tendsto reduce several strength values of the coated paper, particularly wetpick strength and web rub.

To provide adequate coating strength, the alkali-swellable or solublelatex will generally comprise from 2% to 50% of the binder component ofthe coating composition, and preferably from about 20% to about 40%. The

alkali-swellable or soluble latex of the present invention may be anypolymer to which sufiicient carboxyl groups may be attached to producestrong acid. For reasons of economy and polymer strength, acryliccopolymers, styrene-butadiene copolymers or vinyl acetate copolymershave been the commercially available polymers found useful as thealkali-swellable or alkali-soluble latex. The nonalkali-swellable ornon-alkali-soluble latex of the adhesive component may be copolymers ofstyrene-butadiene, styrene-isoprene, styrene-vinyl acetate, acrylics,vinyl acetates, vinyl chloride-acrylic, styrene-nitrile, ornitrileacetate. The pigment component may be any of the commonly usedpaper coating pigments, including domestic and foreign clays, hydratedalumina, calcium carbonate, or combinations thereof.

In the following example the teachings of the prior art as representedby the above-cited patents, U.S. Pats. 3,513,121 and Canadian Pat.862,188, were applied to the casting coating of paper.

EXAMPLE I A coating composition was made by adding the followingingredients under agitation to 200 parts by weight of water: 100 partsby dry weight of a paper-coating quality clay, 0.6 parts by dry weightof sodium acid pyrophosphate and 0.2 parts by dry weight of an anionic,watersoluble polymer (sold commercially by the NOPCO Chemical Divisionof the Diamond Shamrock Company under the designation Nopcosant L) bothas dispersants for the clay, 1 part by dry weight of a defoamer forlatex (sold commercially by Colloids, Inc. under the designation Colloid680), and 0.5 part by dry weight of a nonionic surface active agent as astabilizer. Then were added binder components consisting of 18 parts bydry weight of an alkali insoluble and unswellable latex (13-15, a softacrylic copolymer manufactured by Rohm and Haas) and 7 parts by dryweight of an alkali-swellable latex (E-503, an emulsion of an acryliccopolymer manufactured by Rohm and Haas). The alkali-insoluble latex wasadded to the coating composition prior to the addition of thealkali-swellable emulsion.

The alkali-swellable emulsion was received from the supplier at a pH of2. It was maintained at this pH until after it was added to the othercoating ingredients, in order to prevent difiiculty in incorporating thelatex into the coating. If the alkali-swellable latex had been thickenedby the addition of alkali prior to the time the latex was added to thecoating it would have coalesced and become so viscous that it could beworked into the coating only with great difliculty.

Immediately prior to the addition of the unmodified alkali-swellablelatex to the coating composition, the pH of the composition was 5.6.Addition of the 7 parts by dry weight of the unmodified alkali-swellablelatex reduced the pH of the entire coating composition to 5.0. This pHincreased slightly to 5.15 when 0.13 part by dry weight of polyethylenewas added as a release agent for the coating.

After these ingredients had been thoroughly mixed, 1.51 parts by weightof ammonium hydroxide (28% ammonia) were added to the coatingcomposition as an alkaline thickener for the alkali-swellable emulsion.The addition of the ammonia caused the coating to thicken markedly, andraised the pH of the coating to 8.5. Forty additional parts by weight ofwater brought the coating solids to 41.6%.

This coating composition was applied to a conventional base paper andthe coated surface was pressed against a casting drum, the surface ofwhich was maintained at a temperature of 240 F. After the coated paperwas dry enough to release from the casting drum, it was removed fromcontact with the drum and subsequently examined for surface quality. Thecoated paper surface exhibited a mottled, uneven appearance, indicatingthat it had not cast properly against the finishing drum. The coatedsurface had a gloss reading of 57 at a 75 angle on a Hunter Glossmeter.

The following examples illustrate the advantages of the presentinvention in comparison to the prior art.

EXAMPLE II A slurry was prepared from parts by weight of English coatingclay, 100 parts by weight of water and 0.7 parts by weight of sodiumtetraphosphate as a dispersant for the clay. This mixture was slurriedfor 20 minutes, following which were added 0.25 part by dry weight of adefoamer, Drew L-50l, manufactured by Drew Chemical Company, and 0.13part by dry weight of polyethylene as a lubricant.

To this composition were added 15 parts by dry weight of a non-alkaliswellable or soluble latex, B-15, a soft acrylic copolymer manufacturedby Rohm and Haas Company; and 5 parts by dry weight of an alkali-solublelatex, ASE 95, an acrylic emulsion copolymer manufactured by Rohm andHaas Company.

The alkali-soluble latex was received from the supplier at a pH of 3.4and was added directly to the coating composition without modification.When the emulsion was added to the coating composition no appreciablethickening of the coating was observed. After this addition, the entiremixture was maintained under continuous agitation until a homogeneouscomposition was obtained. This coating composition had a pH of 5.2, itssolids content was 41.4%, electric water retention (EWR) time was 1.25seconds and viscosity at 100 r.p.m. was 480 centipoises. In this and thefollowing examples, viscosity was measured with a Brookfield ModelRVF-100 viscometer using the #5 spindle. Water retention was determinedby measuring the electrical conductivity through a sheet of water leafpaper pressed against the coating. As the water migrates from thecoating into the sheet of paper, the conductivity increases. The valuesgiven represent the time required for the conductivity to reach 0.5milliamp.

The above-described coating was applied by means of an air knife coaterto one side of an 8 point Cover Bristol bodystock which had previouslybeen surface sized on the side to be coated. The coating was applied inan amount equal to 14 pounds per ream of 3300 square feet. Web speedduring the coating and casting operations was feet per minute.

Following the application of the coating, the Web passed from thecoating station past a series of infra-red heaters which served to drythe coated sheet partially prior to reaching the casting drum, but whichproduced no gelation or coalescence of the coating prior to the entry ofthe web into the casting nip. The web passed through a nip comprised ofa backing roll and a chromium-surfaced casting drum with a four footdiameter. In the nip, a vigorously boiling pool of water was maintainedat 220 F. This boiling pool produced a large amount of steam which roseabove the nip entrance. After passing through the steam, boiling pooland casting nip itself, the coated paper surface cast firmly against thedrum surface. As the drum revolved, the paper was maintained in intimateadherent contact with the drum surface for the greater portion of arevolution around the drum. The drum, heated to 260 F. dried the paperto a total moisture content of approximately 4% after the paper hadremained in contact with the drum surface for approximatelythree-quarters of a revolution of the drum. At that point, the coatedweb released cleanly from the drum and was wound up. The cast coatedpaper thus produced had excellent qualities of gloss and sparkle to thenaked eye, and had a gloss reading of 75 when measured at a 75 angle ona Hunter Glossmeter. Microscopic examination of the coated surfaceshowed that the coating was uniform and continuous. The coating wasfound to be substantially free from pits or other surface defects.

7 EXAMPLE III A slurry was prepared by combining 55 parts by weight ofwater with, by dry weight, 0.3 part tetrasodium pyrophosphate and 100parts English coating clay. After slurrying for 20 minutes, there wereadded by dry weight 0.025 part of a defoamer, 15 parts of a non-alkaliswellable latex, the soft acrylic copolymer described in Example II, andfinally 10 parts of an alkali-soluble latex, ASE 75, an acrylic emulsioncopolymer manufactured by Rohm and Haas Company.

This coating composition had a pH of 5.8. Its electric water retention(EWR) was 4.8 seconds, and its viscosity at 100 rpm. was 302centipoises. The coating had a solids content of 56%. Under theoperating conditions of Example II, it was applied to a casein surfacesized bodystock in an amount equal to 13 pounds per 3300 square feet. Nogelation of the coating occurred although the coated surface was heatedto approximately 200 F. between the coating station and the castingdrum, as an aid to drying. The coated paper was pressed against theheated casting drum, which was maintained at a temperature of 264 F. Itcast well against the drum, at an operating speed of 200 feet per minuteand when removed from the finishing surface of the casting drum thecoated paper exhibited at 75 a gloss reading of 88.

EXAMPLE IV Employing the same basic formula and operating conditions setforth in Example III, the ASE 75 alkali-swelL able emulsion was replacedby ASE 60, an acid-containing, acrylic emulsion copolymer, manufacturedby Rohm and Haas Company. This alkali swellable material, when used inthe same minor proportion (40% of the binder component) to the majorproportion of non-alkali swellable latex (B1560% of the bindercomponent) as in Example III, produced a cast sheet with excellentsurface characteristics and a 75 gloss reading of 91.

In further experiments, other alkali-swellable or soluble polymers wereemployed instead of the ASE series of materials used in the examplesalready described. The alkali-swellable emulsion of acrylic copolymer(E503 manufactured by Rohm and Haas) which was unsuccessfully tried inExample I when used according to manufacturers instructions, proved tobe a highly desirable binder component in a cast coating compositionwhen employed in an unmodified acidic state.

'Poly(carboxyl acid) thickener (Tylac 9504 manufactured by StandardBrands), alkali-soluble poly(vinyl acetate), poly(vinyl acetate)-acryliccopolymer and modified styrene-butadiene were each used individually inconjunction with the control formulation of Example III, including thesame non-alkali swellable binder material. Each of these formulationscast well and produced sheets with excellent visual characteristics.

In further experiments performed under the conditions of Example III,the B-15 acrylic copolymer was removed and other latices substituted inthe same proportion relative to the ASE-75 alkali-swellable emulsion.Several styrene-butadienes were each combined individually in a castablecoating composition with the alkaliswellable emulsion. These includeduncarboxylated styrene butadiene (Dow Chemical Companys 612), slightlycarboxylated styrene-butadiene (Dows 620), and a high styrene tobutadiene copolymer (Standard Brands Tylac 5071).

Other types of polymers employed as the non-alkali swellable componentin the same control formula were styrene-isoprene, styrene-acetate,polyacrylics, acrylicstyrene, poly(vinyl acetate) copolymers,poly(-vinyl acetate) terpolymers, vinyl chloride-acrylics,nitrice-acetate, and styrene-nitrile. All of these copolymers weresuccessfully utilized as the major, non-alkali swellable bindercomponent of a cast coating composition containing a minor binderproportion of an alkali swellable latex.

8 EXAMPLE v To 55 parts by weight of water were added by dry Weight 0.3part by Weight of tetrasodium pyrophosphate, parts of paper coatingclay, 0.25 part of a defoamer, 0.25 part of polyethylene as a releaseagent, and, as a binder component, 18 parts of a non-alkali swellablelatex, the soft acrylic copolymer described in Example II and 2 parts ofan alkali soluble, poly(vinyl acetate) latex, (RV 2255B manufactured bythe General Latex & Chemical Company). This coating composition, with ahigh ratio of non-alkali soluble to alkali soluble binder components,had the following characteristics: pH 6.8, EWR 3.8 seconds, andBrookfield viscosity at 100 rpm, centipoises. This coating was appliedat 57.6% solids, did not gel when heated prior to the casting drum, andcast Well against the heated drum. The coated paper released cleanlyfrom the drum when dry. It exhibited good visual gloss.

EXAMPLE VI To 100 parts by weight of water were added, by dry weight,100 parts of domestic, coating-quality clay and 0.5 part of a dispersantfor the clay. This mixture was slurried for 20 minutes, at which point0.25 part of a defoamer, 10 parts by dry Weight of the non-alkaliswellable latex of Example II, 025 part of polyethylene as a releaseagent, and 10 parts by dry weight of the alkaliswellable latex ofExample II were added. This composition had a pH of 4.65, a viscosity at100 rpm. of 480 centipoises and an EWR of 1.5 seconds. It was applied at42.4% solids to a casein surface sized Cover Bristol bodystock in anamount equal to 13 pounds per 3300 square feet. The coated sheet castwell and had a finished appearance that was satisfactory althoughsomewhat less desired than those of the preceding examples where lessalkali-soluble latex was employed.

EXAMPLE VII Due to the basic pH of several components normally presentin paper coating compositions, all-latex cast coating compositions towhich an unmodified alkali-swellable or alkali soluble latex has not yetbeen added may have a pH at or above the neutral 7.0. A sudden change inthe viscosity and homogeneity of the coating, called shocking, may occurwhen a strongly acidic, unmodified, alkali-swellable or soluble latex isadded. In order to avoid this problem, it is desirable, in someformulations and under some operating conditions, to add a bufferingingredient which will lower the pH of a neutral or alkaline coatingsomewhat prior to the addition of the unmodified alkali-swellable orsoluble latex. This practice is illustrated as follows:

A slurry of 200 parts by weight of water and 300 parts by dry weight(60% solids) of a domestic, coating-quality clay had a pH of 7.2. Afterthe addition of 1.75 parts by weight of a defoamer (Drew ChemicalCompanys L- 501) and 45 parts by dry weight of the non-alkali swellablelatex of Example II, the pH of the composition was 6.1. The compositionwas then buffered against the addition of the alkali-swellable latex bythe addition of 1 part by dry weight of succinic anhydride (pH 4.2).This lowered the pH of the entire composition to 4.6. Addition of 30parts by dry weight of the alkali-swellable latex of Example III (pH3.0) reduced the final pH of the composition to 4.2. No radical changesin the coating composition were noted when the alkali-swellable latexwas added in this manner.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it is understood thatvarious other changes and modifications thereof will occur to a personskilled in the art without departing from the spirit and scope of theinvention as defined by the appended claims.

9 What is claimed is: 1. A method of cast-coating paper comprising thesteps of:

applying to a paper web an aqueous coating composition in which theadhesive consists essentially of a mixture of latices comprising 50% to98% of latex which is not capable of being swelled or rendered solubleby alkali and 2% to 50% of latex which is capable of being swelled orrendered soluble by alkali, the pH of the coating composition being suchthat the latex capable of being swelled or rendered soluble remains inan unswelled or undissolved condition; and conveying the coated paperweb into a casting nip while maintaining the pH of the coatingcomposition on the web at a level such that the latex capable of beingswelled or rendered soluble remains in .an unswelled or dissolvedcondition. 2. The method in accordance with claim 1 wherein the castingnip contains a pool of boiling water.

3. The method in accordance with claim 1 wherein the latex capable ofbeing swelled or rendered soluble by alkali is taken from the groupconsisting of acrylic copoly- References Cited UNITED STATES PATENTS3,600,215 8/1971 Mervine 11764 C 3,268,354 8/1966 Hain 11764 C 2,759,8478/ 1956 Frost et a1. 11764C 3,681,116 8/1972 Reich et al. 11755 UA3,415,671 12/1968 Rice 11764 R 2,678,890 5/ 1954 Leighton 11764 C2,810,966 10/1957 Bicknell 11764 R WILLIAM D. MARTIN, Primary ExaminerW. R. TRENOR, Assistant Examiner US. Cl. X.R. 117-l55 UA, 156

